This invention relates to wireless microcell distribution systems, and more particularly to the embedding of forward reference and control signals within the cable channel utilized for transporting telephony signals to and from a microcell connected to the cable.
In the past, wireless communication has been established between a wireless transceiver and cell sites which are spaced to be able to receive transmissions from the transceivers and to transmit corresponding telephony signals over landlines to either landline-based telephones or other wireless handsets. The problem with cell sites is in general the cost and siting of cell site towers. In an effort to minimize cost and increase reliability, cable television networks have been utilized to transport the telephony signals, with microcells positioned at various points along the cable to receive signals from the wireless transceivers and transmit signals back to the transceivers in a full duplex operation. In such systems, the cables are typically already in place and carry CATV signals.
In a wireless microcell distribution system, a base station is connected at one end to the telephone network switch and at the other end to. the cable that is used for connecting the microcells, hereinafter referred to as cable microcell integrators. Each of the cable microcell integrators is provided with transmit and receive antennas and transceivers which permit communication with the wireless transceivers. As such, each of the microcells functions as a cell site, with the microcells being interconnected by the cable. This type of system eliminates costly towers and siting by providing a number of microcells spaced out along the same cable that already carries CATV signals. Thus, in the past, existing cable television networks have been adapted to carry telephony signals between a telephone network and a remote transceiver site, the coverage of which defines cells or sectors.
In these systems, the base transceiver acts as the interface between the telephone network and the wireless telephones. To carry the wireless telephony signals over a broad band distribution network, a predetermined bandwidth on the network is typically allocated for this purpose. To most efficiently use a given bandwidth to carry wireless telephony signals between wireless telephones and the telephone network, code, frequency or time division multiplexing is utilized to support code division multiple access, time division multiple access, frequency division multiple access. This requires appropriate base station equipment that acts as the interface with the telephone network and the wireless telephone system.
The base station is typically coupled to a, head end interface converter, the purpose of which is to provide the required number of telephony signals, and both control and reference signals to a coupler which couples these signals to a cable-to-fiber transducer. The cable-to-fiber transducer injects the requisite signals into the cable, with a splitter being provided at a point on the cable to split out CATV signals and the telephony signals. The splitter provides the telephony signals as well as the CATV signals, to a cable microcell integrator connected to the cable, with the microcell having three antennas, one a transmitting antenna and the others a primary and diversity antenna which serve as the receiving antennas.
Control and reference signals are called forward control and reference signals because they are the signals transmitted from the head end interface converter to the microcells to control the associated cable microcell integrators. The forward control signal typically controls frequency channel assignment and the power levels utilized by the microcells, with the forward reference signal provided to permit phase locking of the oscillators in the microcell.
It will be appreciated that the communication between the head end interface converter and each of the microcells is bi-directional. In the forward direction, control over power and gain of each of the microcells is provided through the forward signal to each of the microcells. Likewise, channel assignment for the telephony signals and the CATV signals is provided by forward control signals. Moreover, fault enable and disable control signals are provided in the forward direction to each of the modules. Additionally, transmit enable and disable signals are also provided to the microcells, along with a status monitoring and fault reporting signal. Moreover, software updates for each of the modules can be provided by the forward control signal.
In the reverse direction, signals from the module are transmitted to the head end interface converter which include responses to all of the queries contained in the forward control signals. These reverse control signals are provided back to the head end interface converter at frequencies much below the forward portion of the CATV band.
In the past, these forward control and forward reference signals have been fixed at 52.0 MHz and 52.5 MHz respectively, which are just below the forward position of the CATV band. It will be appreciated that the forward portion of the CATV band typically runs from 52.7 MHz to 800 MHz, with the telephony signals being assigned to a particular Cable TV channel, which in general is 6 MHz wide.
The problem with locating the forward control and reference signals below the forward portion of the CATV band is first the problem of roll off. Roll off occurs when equipment on the cable plant is set to operate within the allocated band. The forward control and reference signals being below this band are attenuated due to the out of band nature of these signals. In particular, couplers and the line amplifiers operate within the forward CATV band and attenuate signals to either side of that band in a roll off which is multiplied with the large number couplers and line amplifiers utilized in the cable plant. While the loss at a single coupler or line amplifier is minimal, when these couplers and amplifiers are cascaded, the roll off effect on the forward control and reference signals is significant.
The second problem with locating the forward control and reference signals below the forward end of the CATV band is a problem called cable tilt. Cable tilt refers to the difference in attenuation with frequency of signals transmitted along the cable. It is noted that the higher the frequency the greater the loss per unit length of cable. For telephony signals at a given frequency channel, the amplitude difference between the reference and control signals below the forward CATV band and the telephony signals within the forward CATV band is significant. One can therefore no longer set the power levels at each microcell to the appropriate levels due to the variable distance between the head end system and the microcells in which the control and reference signals are attenuated differently than the in band telephony signals. The difference in amplitudes with cable length has resulted in the placement of equalizers at each module, with the equalizers adjusted to take into account this difference in amplitude.
Moreover, both the effects of roll off and cable tilt combine to make the imbalance worse. This makes it even more difficult to equalize the signals at each of the microcells.
Rather than placing the forward control and reference signals below the forward CATV band in the subject invention these signals are embedded in the cable channel used for telephony. As a result, there is no disparity in amplitude between the telephony signals and the forward control and reference signals due to the co-location of these signals with the channel. The result is that there is no roll off because all of the couplers and line amplifiers normally used are operating within specification and no additional amplification for the prior out-of-band signals is necessary. Also with the co-location of the forward control and reference signals in the telephony channel, there is no frequency-related or distance-dependant loss. There is no difference in attenuation at a microcell for the control and reference signals and the telephony signals. There is no equalization required at each module which eliminates the problem of cable tilt.
Moreover, it will be appreciated that no extra spectrum is utilized and installation difficulties associated with the provision of equalizers are eliminated.
Importantly, the elimination of cable tilt and roll off by the embedding of the forward reference and control signals means that the cable plant need not be altered in any way to provide for the telephony service. This means that no additional amplifiers or equalizers are required to be inserted into the cable system as is the case when the control and reference signals are located below the forward CATV band.
In summary, In a wireless microcell distribution system, a method is provided for embedding reference and control signals within the channel utilized for carrying the telephony signals, thereby eliminating roll off and cable tilt issues which are the result of providing the reference and control signals below the forward CATV band. The elimination of the signals below the forward CATV band also eliminates additional cable spectrum and installation difficulties which are the result of cable tilt in which frequency dependent amplitudes require compensation through the utilization of equalization for each cable microcell integrator.